My battery life on Mini 4 seems pretty good. But the lag and framerate drops are unacceptable when I've spent this much money on a small tablet. Part of the reason I prefer iOS over Android is because Android often has random lags and hiccups. This is the first iOS device I've owned that behaves this way.
BTW, iOS 9.1 was officially released. I updated it yesterday, and it doesn't improve anything. I know from experience that it's not wise to take a gamble that performance issues on any device will get fixed.
At this point, sadly I think I'll be returning my Mini 4 on Monday.
Phil Schiller lied on stage about iPad Mini 4 being identical to iPad Air 2 but smaller. iPad Mini 4 A8 GPU performance is nearly half that of the iPad Air 2 A8X. No software update is going to fix that.
http://www.apple.com/ipad/compare/